This invention relates to the field of dialyzers typically used in hemodialysis and related medical procedures, and in particular to a design having a lengthened useful life, together with a method for manufacturing the same.
Patients with kidney disease suffer from the adverse effects of toxin build-up in their blood. Dialysis is a process which employs an artificial kidney to remove those toxins. In hemodialysis a dialyzer is used which contains a semi-permeable membrane dividing the dialyzer into two chambers. Blood is pumped through one chamber and a dialysis solution through the second. As the blood flows by the dialysis fluid, separated by the semi-permeable membrane, blood impurities such as urea and creatinine diffuse through the semi-permeable membrane into the dialysis solution by the diffusion, convection and absorption. The electrolyte concentration of the dialysis fluid is set so as to maintain electrolytic balance within the patient.
Dialyzers often use a large number of microfibers encased in a chamber. The chamber is often a hollow cylinder open at both ends. Thousands of hollow semipermeable microfibers carry blood from one end to the opposite end so that blood flows through the microfibers in a first direction. Dialysate ports are also present on opposite ends of the chamber. One port carries dialysate into the chamber, the dialysate flows through the chamber in a countercurrent direction to the blood flow, and the other port carries the dialysate out of the chamber. The solute removal thus takes place across the semipermeable membrane that is the microfiber wall. This design produces a high surface area for solute removal in a relatively low volume device.
One significant challenge is to connect the microfiber interior channels to the blood lines, so that blood flows smoothly from the arterial blood line, into the microfiber interior channels where it can pass into and through the dialyzer chamber, and out the other end of the microfiber interior channels to the venous blood line.
This is done by filling the open-ended cylinder-shaped dialyzer chamber with microfibers extending in the longitudinal direction. Each end of the cylinder is threaded to receive a cap closing the end. The dialyzer is then positioned in a centrifuge to allow rotation about an axis perpendicular to the central longitudinal axis, wherein the axis of rotation extending through the midpoint of the cylinder. A liquid potting material such as epoxy or urethane is then injected into the dialysate ports on each end of the chamber, and the dialyzer is spun in the centrifuge. The centripetal force produced by the rotation in the centrifuge forces the potting material to each end, where it sets and hardens.
The dialysate ports in the cylinder wall near each end of the dialyzer present special challenges. It is desirable for the microfibers to be supported by the cylinder wall throughout their length, or as much of their length as possible, so that the dialysate fluid flow pressure on the microfibers does not distort them. Such distortion can result in their breaking, which allows mingling of blood and dialysate, or can obstruct the flow of blood through the microfiber interior channels.
Support for the microfibers is achieved in some designs by separating the dialysate ports from the dialyzer chambers with a dialysate fluid distribution ring. The dialysate fluid distribution ring is effectively an extension of the dialyzer cylinder wall. The dialysate port accesses the dialyzer chamber by means of an intermediate space defined at the radial inner side by the dialysate fluid distribution ring and at the radially outer side by an outer wall. The dialysate fluid distribution ring thus extends lateral support to the microfibers around the circumference of the microfiber bundle while still enhancing dialysate fluid distribution between the dialysate ports and the chamber.
In practice, this design is quite effective. It has been discovered, however, that the microfibers tend to fail first at a particular location due apparently to an artifact of the potting process. As the dialyzer is spun in a centrifuge to force potting material to each end to encase the microfibers, the surface closest to the axis of spinning rotation assumes a curve. It is believed that this is because the centripetal force exerted on the potting material is in proportion to the distance of the potting material from the axis of rotation, much as the outward force on a merry-go-round is greater at the edge than at the center. In order to maintain constant centripetal force along the potting material surface during the centrifuge operation, that surface assumes a curvature that maintains a constant distance from the axis of centrifuge rotation. That resulting curvature departs slightly from the plane resulting from cutting off the ends of the potting material to expose the microfiber interior channels.
Like the cut-off end of the potting material, the dialysate fluid distribution ring edge has in the past been planar. Because the dialysate fluid distribution ring edge has been planar while the potting material surface has been curved, the width of the gap between the dialysate fluid distribution ring edge and the potting material has varied. This gap represents the space in which the microfiber bundle is unsupported by the dialyzer cylinder wall or by the dialysate fluid distribution ring. The dialysate fluid flow pressure between the microfiber interior channel and the dialyzer chamber interior tends to flex or distort the microfibers. Because pressure produces a force in proportion to the area on which it acts, the force produced at areas of relatively large gap between the dialysate fluid distribution ring edge and the potting material surface is different from the force produced at areas of relatively small gap between the dialysate fluid distribution ring edge and the potting material surface. It has been found that this differential force is responsible for much of the microfiber distortion, rupturing and obstruction seen in the initial stages of dialyzer destruction.
It is desirable for dialyzers to be filled with microfibers in a manner that supports the dialysate fluid flow pressure on the microfiber bundle in a uniform way to avoid differential forces on the microfibers which distorts, ruptures or obstructs them.
The present invention overcomes the deficiencies of existing systems by use of a novel design for the dialyzer dialysate fluid distribution ring. This dialysate fluid distribution ring separates the dialyzer interior chamber from the dialysate ports. The dialysate fluid distribution ring edge distal from the centrifuge axis of rotation assumes a shape defined as an annulus cut by a cylinder perpendicular to the axis of the annulus.